Marking of information on a label, etc. is generally effected by direct printing. The printing is usually carried out by pad printing or screen printing. Pad printing has an advantage in printing on a curved surface because of the elasticity of the pad but is disadvantageous in making a fine pattern with precision. Screen printing also meets difficulty in obtaining a fine pattern with precision due to the limited mesh size of the screen. Besides the poor precision, since printing involves making of a plate for every desired pattern or requires time for setting printing conditions, these methods are by no means suitable for uses demanding real time processing.
Hence, marking by printing has recently been replaced by ink jet marking. Although ink jet marking satisfies the demand for speediness and real time processing which are not possessed by a conventional printing system, the ink to be used, which is jetted from fine nozzles under high pressure, is strictly specified. Unless the specification is strictly met, the ink sometimes causes obstruction of nozzles, resulting in an increase of reject rate. This is the very problem waiting for solution as regards the ink jet system.
In order to overcome the problem, laser marking has lately been attracting attention as a high-speed and efficient marking method and already put to practical use for some uses. Most of the currently developed laser marking techniques comprise irradiating only a necessary area of a substrate with laser light to denature or remove the irradiated area or irradiating a coated substrate with laser light to remove the irradiated coating layer thereby making a contrast between the irradiated area (marked area) and the non-irradiated area (background).
However, in the method of removing a printing ink by laser light irradiation, a contrast between the paper substrate and the printing ink is required, and clear marking cannot be achieved with a lightly colored label. Further, removal of a printing ink, which is nothing less than destruction of the material, causes roughening of the boundary between irradiated and non-irradiated areas, resulting in a failure of obtaining precise marking. It has therefore been suggested to utilize non-destructive color development by laser light irradiation on a label comprising a paper substrate, etc., i.e., thermosensitive color formation. We can find in literature various examples of application of the thermosensitive color formation system (see, e.g., JP-A-53-52442, JP-A-55-11857 and JP-A-59-148695, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but no cases has been reported in which the system is put to practical use.
On the other hand, in using a one-pack printing ink containing a leuco dye as a color former and an acidic substance as a color developer, if the color former and the color developer are previously mixed and allowed to stand in a hydrophilic medium for a long time, the printing ink tends to undergo self-color formation to cause background coloration, which results in a reduction in contrast between a marked area and a non-marked area (background).
In the light of the above-described circumstances, the present inventors previously studied laser light-induced non-destructive color development and developed a printing ink, e.g., a gravure ink, containing a color former, such as a leuco dye, and a color developer therefor. A marking material comprising a substrate having printed thereon a color-forming layer comprising this ink develops a color image on being irradiated with laser light to easily achieve marking on the substrate made of even paper without being accompanied by destruction of the material (see JP-A-5-58031 and JP-A-5-57463).
It turned out, however, that the above-mentioned printing ink sometimes suffers from color formation during ink preparation (dark color formation) or after printing (background color formation) (the dark color formation will hereinafter be included in background color formation) due to the fact that both the leuco dye as a color former and an acidic substance as a color developer exist in the same system. There has still been a need of further improvement for practical use.